1. Field of the Invention
The present invention relates to a display element having a plurality of display units laminated to each other, an electronic paper including the same, an electronic terminal apparatus including the same, a display system including the same, and a method of processing an image in a display element.
2. Description of the Related Art
In recent years, many companies and universities have developed electronic papers. The electronic papers can be applied to portable devices, such as electronic books, sub-displays of mobile terminals, and display units of IC cards. As an example of a display element used for the electronic paper, there is a display element that uses a liquid crystal composition having a cholesteric phase formed therein (cholesteric liquid crystal). The cholesteric liquid crystal has, for example, a semipermanent display retention characteristic (memory property), a clear color display characteristic, a high contrast characteristic, and a high resolution characteristic.
FIG. 11 is a cross-sectional view schematically illustrating the structure of a liquid crystal display element 51 capable of performing full color display using the cholesteric liquid crystal. The liquid crystal display element 51 has a structure in which a blue (B) display unit 46b, a green (G) display unit 46g, and a red (R) display unit 46r are laminated in this order from a display surface. In FIG. 11, the outer surface of an upper substrate 47b serves as the display surface, and external light (solid arrow) is incident on the display surface from the upper side of the substrate 47b. In addition, an observer's eye and a viewing direction (dotted arrow) are schematically depicted above the substrate 47b. 
The B display unit 46b includes a pair of upper and lower substrates 47b and 49b, a blue (B) liquid crystal 43b sealed between the two substrates, and a pulse voltage source 41b that applies a predetermined pulse voltage to the B liquid crystal layer 43b. The G display unit 46g includes a pair of upper and lower substrates 47g and 49g, a green (G) liquid crystal 43g sealed between the two substrates, and a pulse voltage source 41g that applies a predetermined pulse voltage to the G liquid crystal layer 43g. The R display unit 46r includes a pair of upper and lower substrates 47r and 49r, a red (R) liquid crystal layer 43r sealed between the two substrates, and a pulse voltage source 41r that applies a predetermined pulse voltage to the R liquid crystal layer 43r. A light absorbing layer 45 is provided on the rear surface of the lower substrate 49r of the R display unit 46r. 
The cholesteric liquid crystal used for each of the B, G, and R liquid crystal layers 43b, 43g, and 43r is a liquid crystal mixture of nematic liquid crystal and a relatively large amount of additive, for example, several tens of percent by weight of additive (which is also called a chiral material). When a relatively large amount of chiral material is added to the nematic liquid crystal, it is possible to strongly twist nematic liquid crystal molecules into a helical shape, thereby forming a cholesteric phase. The cholesteric liquid crystal is also called chiral nematic liquid crystal.
The cholesteric liquid crystal has bistability (memory property). It is possible to change the cholesteric liquid crystal to a planar state, a focal conic state, or an intermediate state between the planar state and the focal conic state by adjusting the intensity of an electric field applied to the liquid crystal. Once the cholesteric liquid crystal is changed to the planar state or the focal conic state, the cholesteric liquid crystal stably maintains its state even when no electric field is applied.
The planar state is obtained by applying a predetermined high voltage between the upper and lower substrates 47 and 49 to apply a strong electric field to the liquid crystal layer 43 and then rapidly reducing the electric field to zero. The focal conic state is obtained by applying, for example, a predetermined voltage that is lower than the above high voltage between the upper and lower substrates 47 and 49 to apply an electric field to the liquid crystal layer 43 and then rapidly reducing the electric field to zero. The intermediate state between the planar state and the focal conic state is obtained by applying, for example, a voltage that is lower than that used to obtain the focal conic state between the upper and lower substrates 47 and 49 to apply an electric field to the liquid crystal layer 43 and then rapidly reducing the electric field to zero.
Next, the display principle of the liquid crystal display element using the cholesteric liquid crystal will be described with reference to FIGS. 12A and 12B, using the B display unit 46b as an example. FIG. 12A depicts the arrangement of liquid crystal molecules 33 of the cholesteric liquid crystal when the B liquid crystal layer 43b of the B display unit 46b is in the planar state. FIG. 12B depicts the arrangement of the liquid crystal molecules 33 of the cholesteric liquid crystal when the B liquid crystal layer 43b of the B display unit 46b is in the focal conic state.
Depicted as FIG. 12A, the liquid crystal molecules 33 in the planar state sequentially rotate in the thickness direction of the substrates to form a helical structure, and the helical axis of the helical structure is substantially vertical to the surfaces of the substrates. In the planar state, light having a predetermined wavelength corresponding to the helical pitch of the liquid crystal molecules is selectively reflected from the liquid crystal layer. When the average refractive index of the liquid crystal layer is n and the helical pitch is p, a wavelength λ where the highest reflectance is obtained is represented by λ=n·p.
Therefore, in order to selectively reflect blue light from the B liquid crystal layer 43b of the B display unit 46b in the planar state, the average refractive index n and the helical pitch p are determined such that the wavelength λ is, for example, 480 nm. The average refractive index n can be adjusted by selecting a liquid crystal material and a chiral material, and the helical pitch p can be adjusted by adjusting the content of the chiral material.
Depicted as FIG. 12B, the liquid crystal molecules 33 in the focal conic state sequentially rotate in the in-plane direction of the substrates to form a helical structure, and the helical axis of the helical structure is substantially parallel to the surfaces of the substrates. In the focal conic state, the selectivity of the B liquid crystal layer 43b with respect to a reflection wavelength is lost, and the B liquid crystal layer 43b transmits most of incident light. The transmitted light is absorbed by the light absorbing layer 45 that is provided on the rear surface of the lower substrate 49r of the R display unit 46r. As a result, dark (black) display is obtained.
As described above, it is possible to control the reflection and transmission of light by adjusting the arrangement state of the cholesteric liquid crystal molecules 33 twisted in the helical shape. Similar to the B liquid crystal layer 43b, the cholesteric liquid crystal that selectively reflects green or red light in the planar state is injected into the G liquid crystal layer 43g and the R liquid crystal layer 43r to manufacture the liquid crystal display element 51 capable of performing full color display.
FIG. 13 is a diagram illustrating an example of the reflection spectrum of each of the liquid crystal layers 43b, 43g, and 43r in the planar state. In FIG. 13, the horizontal axis indicates the wavelength (nm) of reflected light, and the vertical axis indicates reflectance (with respect to a white board; %). The reflection spectrum of the B liquid crystal layer 43b is represented by a curved line linking symbols ▴ in FIG. 13. Similarly, the reflection spectrum of the G liquid crystal layer 43g is represented by a curved line linking symbols ▪, and the reflection spectrum of the R liquid crystal layer 43r is represented by a curved line linking symbols ♦ in FIG. 13.
Depicted as FIG. 13, in the reflection spectrums of the liquid crystal layers 43b, 43g, and 43r in the planar state, R has the longest center wavelength, followed by G and B. Therefore, the liquid crystal layer 43r has the largest helical pitch of cholesteric liquid crystal, followed by the liquid crystal layers 43g and 43b. Thus, it is necessary to adjust the content of the chiral material in the cholesteric liquid crystal in the liquid crystal layers 43b, 43g, and 43r such that the liquid crystal layer 43b has the largest amount of chiral material, followed by the liquid crystal layers 43g and 43r. 
In general, as a reflection wavelength is decreased, it is necessary to reduce the helical pitch of the liquid crystal molecules by strongly twisting. Therefore, the content of the chiral material in the cholesteric liquid crystal is increased. In addition, generally, as the content of the chiral material is increased, a driving voltage tends to be increased. Further, a reflection bandwidth Δλ is increased as the refractive index anisotropy Δn of the cholesteric liquid crystal is increased.
Patent Document 1: JP-A-2004-219715
Patent Document 2: JP-A-2002-139746
However, in the liquid crystal display element using the cholesteric liquid crystal, when a still picture is displayed for a long time, ‘image sticking’, which is a phenomenon where a faint outline of a previously displayed image remains visible on the screen when the image is changed, occurs. It is estimated that the image sticking is caused by various factors, such as water, ionic impurities, and the affinity between liquid crystal and a substrate interface. In order to remove the image sticking, a high degree of refining of a material or high stability of an interface state is needed. In addition, for example, as a method of preventing the image sticking, a timer or an optical sensor is provided in a liquid crystal display element to detect a continuous operation time or that the liquid crystal display element is placed in a dark environment, thereby making the entire screen in a standby state (off display). However, in this method, it takes time for the screen to return from the standby state (redisplay). Therefore, when it is necessary to rapidly view a display image, the convenience of the liquid crystal display element significantly deteriorates.
In general, as an environmental temperature is increased, the degree of image sticking is increased. Therefore, Patent Document 1 discloses a method of preventing image sticking by displaying an image sticking prevention pattern causing the entire screen to be black to change liquid crystal to a focal conic state when a temperature sensor detects a temperature that is equal to or higher than a predetermined value. However, when the image sticking prevention pattern is displayed on the display screen, a previously displayed image is temporarily removed. Therefore, the convenience of a display element significantly deteriorates.
Patent Document 2 discloses a method of reducing power consumption by dividing a common electrode into segment electrodes for each digit in 7-segment monochrome display. In addition, Patent Document 2 discloses a structure that initializes a display element in order to prevent image sticking. However, Patent Document 2 discloses only the 7-segment monochrome display, but does not disclose a dot matrix display element capable of performing color display.